Iron electrode employing a polyvinyl alcohol binder

ABSTRACT

The present invention provides one with an iron electrode employing a binder comprised of polyvinyl alcohol (PVA) binder. In one embodiment, the invention comprises an iron based electrode comprising a single layer of a conductive substrate coated on at least one side with a coating comprising an iron active material and a binder, wherein the binder is PVA. This iron based electrode is useful in alkaline rechargeable batteries, particularly as a negative electrode in a Ni—Fe battery.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority as a continuation application to U.S.application Ser. No. 14/170,043, filed Jan. 31, 2014; which in turnclaims priority to U.S. Application No. 61/898,115, filed Oct. 31, 2013;and U.S. Application No. 61/759,777, filed Feb. 1, 2013, whichapplications are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is in the technical field of energy storagedevices. More particularly, the present invention is in the technicalfield of rechargeable batteries employing an iron electrode.

State of the Art

Iron electrodes have been used in energy storage batteries and otherdevices for over one hundred years. In particular, iron electrodes areoften combined with a nickel-based positive electrode in alkalineelectrolyte to form a nickel-iron (Ni—Fe) battery. The Ni—Fe battery isa rechargeable battery having a nickel(III)oxy-hydroxide positiveelectrode in combination with an iron negative electrode with analkaline electrolyte such as potassium hydroxide.

The Ni—Fe battery is a very robust battery which is very tolerant ofabuse such as overcharge and overdischarge and can have a very longlife. It is often used in backup situations where it can be continuouslytrickle-charged and last more than 20 years.

Traditionally, the iron electrode active material is produced bydissolving pure iron powder in sulfuric acid, followed by drying androasting to produce iron oxide (Fe₂O₃). The material is washed andpartially reduced in hydrogen and partially oxidized to give a mix of Feand magnetite (Fe₃O₄). Additives such as FeS may be added to the activematerial mass. The negative electrode structure is typically that of apocket plate construction wherein the active material is introduced intothe current collector. The current collector is made up of steel stripsor ribbons that are perforated and nickel plated and the strip formedinto a tube or pocket with one end left open for introduction of theactive material (D. Linden and T. Reddy, Editors, “Handbook ofBatteries, Third Edition”, McGraw-Hill, © 2002). Alternatively, fineiron powder can be sintered under a reducing atmosphere to yield asturdy electrode shape.

Both of these methods for producing iron electrodes are expensive, leadto low active material utilization, and poor specific energy. As aresult, Ni—Fe batteries have largely been displaced by other batterytechnologies due to the high cost of manufacturing and low specificenergy. While the technology of preparing iron electrodes is well knownand the current preferred process for making these electrodes is apocket design, pocket design electrodes are not cost effective and arecomplex in manufacturing. Although the theoretical capacity of an ironelectrode is high, in practice only a small percentage of this isachieved due to the poor conductivity of iron oxide. In a pocketelectrode design, loss of contact to the external matrix surface resultsin increased polarization and a drop in cell voltage. To avoid this,large amounts of conductive material such as graphite must be added tothe active material, further increasing cost and lowering energydensity. The industry would be well served by a low cost, high qualityand high performance iron electrode design.

The substrate in an electrode is used as a current conducting andcollecting material that houses the active material (iron) of theelectrode in a mechanically stable design. In current pocket electrodedesigns, the substrate encompasses the active material and holds thematerial between two layers of conductor, therefore requiring twosubstrates per electrode. In this process, pockets are formed byinterlocking two perforated Ni-coated strips into which the activematerial is compressed. While such a design offers long life, the energydensity is poor.

An alternative process utilizes a porous sintered structure of ironpowder, which is filled with iron hydroxide by either an electrochemicalprocess or by impregnation of the pores with an appropriate iron salt,followed by immersion in alkaline solution. Such electrodes suffer frompoor active material loading and corrosion of the iron porous plaqueduring impregnation, leading to limited life.

To address these short-comings, U.S. Pat. No. 4,236,927 describes aprocess whereby iron powder and a reducible iron compound are mixedtogether and sintered into a stable body. This mixture is then sinteredat high temperature to form a plate of desired shape. While thiseliminates the need for a sintered plaque substrate or pockets ofNi-coated steel, it requires high temperature sintering under hydrogenatmosphere. Such processes add considerable complexity and cost involume manufacturing.

Other forms of electrode production are known in the art, particularlyelectrodes of a pasted construction. This type of electrode typicallyincorporates a binder with the active material, which can then be coatedonto a two or three dimensional current collector, dried, and compactedto form the finished electrode.

U.S. Pat. No. 3,853,624 describes a Ni—Fe battery incorporating ironelectrodes employing a metal fiber structure which is loaded withsulfurized magnetic iron oxide by a wet pasting method. The plates areelectrochemically formed outside the cell to electrochemically attachthe iron active material to the plaque structure. Such a process inunwieldy in high volume manufacturing and adds to product cost.

U.S. Pat. No. 4,021,911 describes an iron electrode wherein the ironactive mass is spread onto a grid and rolled and dried. The electrode isthen treated with an epoxide resin solution to form a solid reinforcingfilm-like layer on the electrode surface. However, it can be expectedthat such a surface film would contribute to an insulating nature to theelectrode surface, significantly increasing charge transfer resistanceand lowering the cell's ability to sustain high charge and/or dischargerates.

Similarly, PTFE has been proposed as a binder system for paste typeelectrodes for alkaline batteries. U.S. Pat. No. 3,630,781 describes theuse of a PTFE aqueous suspension as a binder system for rechargeablebattery electrodes. However, to maintain the PTFE powder in suspension,it is necessary to add surfactants to the suspension, which must beremoved from the resultant electrode by extensive washing, adding costand complexity to the manufacturing process. An alternative approach fora PTFE-bonded electrode is described in U.S. Pat. No. 4,216,045 usingfluorocarbon resin powder to form a sheet which can be attached to aconductive body. However, the use of PTFE results in a water-repellentsurface, which while beneficial in a recombinant battery such as NiCd orNiMH, is detrimental to the performance of a flooded Fe—Ni battery wheregood contact between the electrode and electrolyte is beneficial.

Pasted electrodes using various binders have been proposed for alkalineelectrodes, most particularly for electrodes employinghydrogen-absorbing alloys for NiMH batteries (for example U.S. Pat. No.5,780,184). However, the desired properties for these electrodes differsignificantly from those desired for a high capacity iron electrode. Inthe case of the MH electrode, high electrode density (low porosity) isrequired to maintain good electrical contact between the alloy particlesand to facilitate solid-state hydrogen diffusion in the alloy. Bycontrast, high porosity is desirable for iron electrodes due to the lowsolubility of the iron oxide species. Hence, binder systems developedfor other types of alkaline electrodes have not been optimized for Fe—Nibatteries and hence have not found commercial application.

Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer prepared bypartial or complete hydrolysis of polyvinyl acetate to remove acetategroups. Due to its excellent resistance to alkaline environments, PVAhas been proposed for use in separators for alkaline batteries (e.g.U.S. Pat. No. 6,033,806). Additionally, PVA has been employed as abinder material for certain alkaline battery electrodes, most notably,nickel hydroxide electrodes. However, these electrodes are characterizedby a three dimensional structure such as a foam or felt substrate thatprovides mechanical stability to the finished electrode. Therefore, itis not critical to form a fibrous polymer network to stabilize theactive material within the electrode structure.

PVA has generally not been found to be an effective binder in electrodestructures that rely on a single substrate material such as nickelplated strip (NPS), expanded metal, or wire mesh. This is because of therelatively poor binding properties relative to more fibrous polymerssuch as PTFE. PVA does not provide sufficient binding force to preventpremature shedding of active material and delamination from thesubstrate. For these reasons, more fibrous binders are typicallyemployed, most notably PTFE. However, PTFE suffers from severaldrawbacks. Since PTFE is not water soluble, it must be introduced intothe paste in a colloidal suspension. Such a suspension is unstable andcan flocculate, rendering the suspension unusable. A surfactant is usedto maintain the PTFE in a colloidal suspension, and such a surfactantcan cause foaming during processing and must be completely removed fromthe electrode prior to cell assembly. Similarly, the suspension canstratify, requiring regular stirring of stored material. A furtherproperty of PTFE as a battery electrode binder is that it imparts ahydrophobic nature to the electrode surface. While this may be adesirable property in batteries requiring gas recombination, such asNiCd or NiMH, it is undesirable in a Ni—Fe battery, where suchhydrophobicity may hinder access of the electrolyte to iron activematerial. Other binders have been used in alkaline batteries such asvarious rubbers, but these materials are generally not water soluble,requiring the use of organic solvents, adding cost and complexity tomanufacturing.

PVA has recently been proposed as a component to a binder system forlithium ion batteries employing anode materials that are subject tolarge volume changes, but requires the addition of polyurethane toprovide semi-interpenetrating polymer network (U.S. Pat. No. 7,960,056).

The object of this present invention is to provide a high quality andlow cost iron electrode that overcomes the limitations of currentstate-of-the-art pocket and/or sintered iron electrodes.

SUMMARY OF THE INVENTION

The present invention provides one with a novel iron electrodecomprising a Polyvinyl alcohol (PVA) binder. In one embodiment, the ironelectrode is prepared using a continuous coating process. Specifically,the invention comprises an iron based electrode comprising a singlelayer of a conductive substrate coated on at least one side with acoating comprising an iron active material and a binder, wherein thebinder is PVA. This iron based electrode is useful in alkalinerechargeable batteries, particularly as a negative electrode in a Ni—Febattery.

Among other factors, it has been discovered that the PVA is asurprisingly good binder for the preparation of iron electrodes coatedonto a single substrate such as perforated foil, expanded metal, ormesh. Specifically, the present invention provides a paste style ironelectrode utilizing a single conductive substrate to enable a highcapacity iron electrode for use in rechargeable battery systemincluding, but not limited to, Ni—Fe, Ag—Fe, Fe-air, or MnO₂—Fe.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is a perspective view of a coated iron electrode of the presentinvention comprising a PVA binder;

FIG. 2 is a side view and cross-section view of an iron electrode coatedon both sides of the substrate in accordance with the present invention;

FIG. 3 is a perspective view of a current pocket iron electrode; and

FIG. 4 is a side view and a cross-section view of a current pocket ironelectrode.

FIG. 5 shows cycling data for cells with different concentrations of PVAin the iron electrode.

FIG. 6 is discharge capacities for Ni—Fe cells with iron electrodeshaving varied nickel and iron content.

FIG. 7 is discharge capacities for Ni—Fe cells with iron electrodeshaving varied sulfur content.

DETAILED DESCRIPTION OF THE INVENTION

The invention comprises an iron electrode comprised of a single, coatedconductive substrate employing a PVA binder to affix the active materialto the substrate.

In the present invention, a single layer of substrate is used. Thissingle layer acts as a carrier with coated material bonded to at leastone side. The substrate may be a thin conductive material such asperforated metal foil or sheet, metal mesh or screen, woven metal, orexpanded metal. The substrate may also be a three-dimensional materialsuch as a metal foam or metal felt. In one embodiment, a nickel platedperforated foil has been used.

The coating mixture is a combination of PVA binder and active materialsin an aqueous or organic solution. The mixture can also contain otheradditives such as pore formers, conductive additives such as carbon,graphite, or Ni powder, and reaction promoting additives such as sulfurand sulfur bearing materials such as FeS, Mgs and BiS. Pore formers canbe incorporated to enhance electrode porosity. The PVA binder providesadhesion and bonding between the active material particles, both tothemselves and to the substrate current collector. Use of a binder tomechanically adhere the active material to the supporting singlesubstrate eliminates the need for expensive sintering or electrochemicalpost-treatment.

It has been discovered that there are several advantages to employingPVA as a binder in an iron electrode of the present invention versusconventional binders. PVA is readily water soluble, simplifying themanufacturing process by allowing for direct addition of a PVA solutionto the active material mix and eliminating issues associated with shelflife common with PTFE binders. This property permits ready use in acontinuous coating process. PVA does not impart a hydrophobic nature tothe electrode surface, insuring good contact between the active materialand the alkaline electrolyte. It has also been found that PVA minimizesany increase in cell resistance and offers the highest mAh/g capacitywhen used in an iron electrode.

PVA can be added to the active material paste in the form of aconcentrated solution or in powder form. PVA that is hydrolyzed between98.5 and 100% is preferred in one embodiment. A most preferredembodiment uses PVA that is hydrolyzed between 99.0 and 100%.Furthermore, the PVA has a 4% water solution viscosity between 3-70 cPat 20° C. In a preferred embodiment, the viscosity of a 4% watersolution of the PVA is between 20-40 cP at 20° C. In a most preferredembodiment, the viscosity of a 4% water solution of the PVA is between27-33 cP at 20° C. Concentrations of PVA in the final paste formulationare 1 to 10% by total weight. Preferred concentrations of PVA are in therange of 1 to 5% and a most preferred concentration of PVA in the pasteis between 2.5 to 4%. Lower concentrations of PVA do not providesufficient binding of the active material, while higher concentrationsresult in an increase in electrode electrical resistance, degrading theperformance of the battery under high current loads.

While PVA is not generally considered an acceptable binder forelectrodes employing a single substrate, the unique properties of thepasted iron electrode of this invention enable its use as a binder.During electrochemical cycling of the iron electrode, iron is convertedto iron oxides and iron hydroxides which are only very sparingly solublein the electrolyte. Therefore, these reactions occur at the surface ofthe iron particles. During charge, as the iron oxides and ironhydroxides are reduced back to iron metal, the small iron particleseffectively fuse together, providing strong mechanical binding betweenactive material particles. Thus, unlike conventional battery electrodesthat undergo mechanical swelling and shrinking which result in physicaldegradation of the electrode over time, the iron electrode physicalstrength improves with charge/discharge cycling. It is this distinctionthat enables the use of PVA as a binder for an iron electrode, andallows one to successfully take advantage of PVA and its desirableproperties, as discussed above.

The active material for the mix formulation is selected from ironspecies that can be reversibly oxidized and reduced. Such materialsinclude iron metal, iron oxide materials and mixtures thereof. The ironoxide material will convert to iron metal when a charge is applied. Asuitable iron oxide material includes Fe₃O₄. A preferred form of iron ishydrogen reduced with a purity of about 96% or greater and having a 325mesh size. In addition, other additives may be added to the mixformulation. These additives include but are not limited to sulfur,antimony, selenium, tellurium, bismuth, tin, and metal sulfides andconductivity improvers such as nickel.

Sulfur as an additive has been found to be useful in concentrationsranging from 0.25 to 1.5% and higher concentrations may improveperformance even more. Nickel has been used as a conductivity improverand concentrations ranging from 8 to 20% have been found to improveperformance and higher concentrations may improve performance even more.

Turning to the figures of the drawing, FIG. 1 is a prospective view of acoated iron electrode. The substrate 1 is coated on each side with thecoating 2 comprising the iron active material and binder. This isfurther shown in FIG. 2. In FIG. 2, the substrate 1 is coated on eachside with the coating 2 of the iron active material and binder. Thesubstrate may be coated continuously across the surface of thesubstrate, or preferably, as shown in FIGS. 1 and 2, cleared lanes ofsubstrate may be uncoated to simplify subsequent operations such aswelding of current collector tabs.

FIGS. 3 and 4 of the drawing show a conventional pocket iron electrode.In FIG. 3, the two substrates 21 and 22 are shown to form the pocketwhich holds the iron active material. In FIG. 4, the iron activematerial 23 is held between the two substrates 21 and 22.

ILLUSTRATIVE EXAMPLES Paste Preparation

A water based paste comprised of hydrogen reduced iron powder (325 meshsize), 16% nickel powder #255, 0.5% elemental sulfur powder(precipitated, purified) and the appropriate amount of binder wasprepared using a digital stirring device and 3-wing stirring bladeoperating at 1300 RPM for 10-15 minutes. Deionized water was added tothe mixture to create a paste with a viscosity between 120,000-130,000cP.

Example 1

A series of iron electrodes were prepared by impregnating nickel foamwith various pastes comprising several different binder compositionsdescribed in Table 1. The discharge capacities of the individual cellsprepared from these electrodes were measured and plotted against theamount of iron in the anode in FIG. 5. The effect of rate on capacitywas evaluated by discharging the cells at multiple rates of C/10, C/5,C/2, and 2C where C represents the current required to discharge thecell in one hour.

TABLE 1 Cell # Binder Binder g of iron 1 1% CMC 1% PTFE 6.4 2 1% PVA 1%PTFE 8.5 3 1% CMC 1% AL-2002 latex 7.9 4 1% CMC 1% AL-3001 latex 7.4 51% PVA 1% AL-1002 latex 8.3

Since the binder can contribute to electrode resistance, it is desirableto employ a binder that minimizes an increase in cell resistance andoffers the highest mAh/g capacity. Comparing the 2 C capacities of theNi—Fe batteries, the best results at 2 C discharge rate were obtained incells employing PVA as a binder.

Example 2

Water based pastes (Table 2) were applied to a 1.63″ wide nickel-platedperforated strip with 2-mm perforations by feeding the strip fed throughthe top of an open-bottomed pot attached to a doctor-blade fixture witha gap width set to 0.068″. The paste mixture is poured into the pot andthe perforated strip is pulled down at a rate of 2.7 ft/min coating theperforated strip with the paste mixture. Segments ranging 4-5″ are cutfrom the coated strip and placed into a drying oven at 150° C. for 20minutes.

TABLE 2 PVA concentration Iron in electrode Capacity Sample (%) (g)(mAh/g Fe) 1 3.5 8.3 117 2 3.5 8.45 116 3 3.5 11.4 112 4 5 8.25 89 5 710.1 69 6 9 8.55 8

After drying the coated strips were cut to a standard length of 3″ andthen compressed to thickness to achieve a porosity of approximately 40%.Dried paste mixture was removed from the top 0.25″ of the strip in orderto provide a clean space for a stainless steel tab to be spot-welded.

A series of continuously coated iron electrodes were prepared by coatingperforated NPS with an aqueous mixture of iron powder, nickel powder asa conductivity aid, elemental sulfur and employing PVA as a binder.Multiple levels of PVA were employed in the mixes to evaluate the effectof binder concentration on mechanical stability of the electrode andrate capability of the electrode. At concentrations below 2 weightpercent PVA, the physical integrity of the electrodes was unacceptable.Concentrations of binder above about 5 weight percent showed a sharpdrop in discharge capacity, most likely due to increased electroderesistance and possibly masking of the active material from theelectrolyte interface. Data for cells with varying levels of PVA issummarized in Table 2.

Example 3

A 10 wt % solution of PVA (Elvanol 7130) preheated to between 120-125°F. was added to a jacketed container with iron powder (325 mesh), nickelpowder #255, and sulfur preheated to 120° F. This mixture was stirredfor 30 minutes at 120° F. The solid component mixture of this paste was80% iron, 16% nickel, 0.4% sulfur, and 3.5% PVA. Viscosity measurementsof the paste had a range of 25000 to 39000 cP immediately after removalfrom the container and after a further 90 seconds, the viscosity rangedfrom 22000 to 31000 cP.

The paste mixture was then transferred to a jacketed holding tankpreheated to 110° F. where it was stirred. The paste was pumped to apaste hopper where a perforated nickel plated steel strip was coated.The coated strip was then passed through a doctor blade to achieve acoating thickness between 0.040-0.050″ and introduced to a verticaldrying oven. The first stage of drying consisted of IR heating at 240°F. for 1.67 minutes followed by heating in a conventional oven at 240°F. for 3.35 minutes. The second drying stage with a residence time of1.7 minutes consisted of forced hot air with a set drying temperature of260° F. The paste temperature exiting the ovens did not exceed 210° F.After cooling, the finished coating was calendared to a thickness of0.025″. Pieces of the coating were cut to size and weighed to obtaincoating porosity. The porosity ranged from 34-43% with a targetedporosity of 38%.

Electrodes from Example 3 were used to construct a Ni—Fe battery. Table3 shows the performance of the iron electrode in comparison to othercommercial Ni—Fe batteries employing pocket plate electrodes.

TABLE 3 Electrode Chinese Chinese of present Cell Seiden TaihangUkrainian Russian Zappworks invention Ah/g 0.095 Ah/g 0.130 Ah/g 0.117Ah/g 0.116 Ah/g — 0.126 Ah/g (powder) Ah/g 0.059 Ah/g 0.076 Ah/g 0.075Ah/g 0.084 Ah/g 0.034 Ah/g 0.105 Ah/g (total electrode) Ah/cm³ 0.199Ah/cm³ 0.203 Ah/cm³ 0.216 Ah/cm³ 0.238 Ah/cm³ 0.099 Ah/cm³ 0.430 Ah/cm³(total electrode) Type of Pocket Pocket Pocket Pocket Pocket Continuousiron plate plate plate plate plate coated electrode (Pasted)

Example 4 Paste Preparation

A water based paste comprised of hydrogen reduced iron powder (325 meshsize), nickel powder #255, elemental sulfur powder (precipitated,purified) and the appropriate amount of binder was prepared using adigital stirring device and 3-wing stirring blade operating at 1300 RPMfor 10-15 minutes. Deionized water was added to the mixture to create apaste with a viscosity between 120,000-130,000 cP. The nickel and ironcontent was varied according to Table 3, the sulfur content was 0.5%,and the binder content was 3.5%.

Water based pastes with varying nickel and iron content (Table 4) wereapplied to a 1.63″ wide nickel-plated perforated strip with 2-mmperforations by feeding the strip fed through the top of anopen-bottomed pot attached to a doctor-blade fixture with a gap widthset to 0.068″. The paste mixture is poured into the pot and theperforated strip is pulled down at a rate of 2.7 ft/min coating theperforated strip with the paste mixture. Segments ranging 4-5″ are cutfrom the coated strip and placed into a drying oven at 150° C. for 20minutes.

TABLE 4 Sample Nickel (%) Iron % 1 8 88 2 12 84 3 16 80 4 20 76

After drying the coated strips were cut to a standard length of 3″ andthen compressed to thickness to achieve a porosity of approximately 40%.Dried paste mixture was removed from the top 0.25″ of the strip in orderto provide a clean space for a stainless steel tab to be spot-weldedonto.

Ni—Fe cells were constructed using electrodes fabricated from the pasteswith varying nickel and iron content. The data is shown in FIG. 6. Thecell performance does not appear to be very dependent upon nickelconcentration in the concentration range between 8-16% but improvedcapacity at high (1 C) and low rates (C/10) is observed for electrodeswith 20% nickel.

Examples 5 Paste Preparation

A water based paste comprised of hydrogen reduced iron powder (325 meshsize), nickel powder #255, elemental sulfur powder (precipitated,purified) and the appropriate amount of binder was prepared using adigital stirring device and 3-wing stirring blade operating at 1300 RPMfor 10-15 minutes. Deionized water was added to the mixture to create apaste with a viscosity between 120,000-130,000 cP. The nickel contentwas 16%, polyvinyl alcohol 3.5%, and the sulfur content was variedbetween 0 and 1.5% with the remainder of the electrode composition beingiron powder.

Water based pastes with varying sulfur content were applied to a 1.63″wide nickel-plated perforated strip with 2-mm perforations by feedingthe strip fed through the top of an open-bottomed pot attached to adoctor-blade fixture with a gap width set to 0.068″. The paste mixtureis poured into the pot and the perforated strip is pulled down at a rateof 2.7 ft/min coating the perforated strip with the paste mixture.Segments ranging 4-5″ are cut from the coated strip and placed into adrying oven at 150° C. for 20 minutes.

After drying the coated strips were cut to a standard length of 3″ andthen compressed to thickness to achieve a porosity of approximately 40%.Dried paste mixture was removed from the top 0.25″ of the strip in orderto provide a clean space for a stainless steel tab to be spot-weldedonto.

Ni—Fe cells were constructed using electrodes fabricated from the pasteswith varying sulfur content. The data is shown in FIG. 7. Increasing thesulfur content of the electrode increases the capacity at the C/10discharge rate until the sulfur content reaches about 1.5% where thereis no further increase in capacity. Increasing the sulfur contentincreased the capacity of the iron electrode even at sulfur contents upto 1.5% at the 1 C and 2 C discharge rates.

In the foregoing examples, the invention Ni—Fe battery used anelectrolyte comprised of sodium hydroxide (NaOH), lithium hydroxide(LiOH), and sodium sulfide (Na₂S). A sintered nickel electrodeimpregnated with nickel hydroxide was used as the positive electrode inthe foregoing examples using the iron electrode of the presentinvention. The separator used in the inventive Ni—Fe battery was a 0.010inch thick polyolefin non-woven mesh. The electrolyte used in theconventional Ni—Fe battery was potassium hydroxide (KOH), and the anodeand cathode was kept electrically isolated using a spacer. The resultsshow a vast improvement in performance characteristics for the inventiveNi—Fe battery.

While the foregoing written description of the invention enables one ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary skill will understand andappreciate the existence of variations, combinations, and equivalents ofthe specific embodiment, method, and examples herein. The inventionshould therefore not be limited by the above described embodiment,method, and examples, but by all embodiments and methods within thescope and spirit of the invention.

1. A method of preparing an iron electrode comprising the steps of i)preparing a paste formulation which comprises an iron active material,sulfur and from 2 to 5 wt % of a polyvinyl alcohol binder; ii) providinga single layer substrate; and iii) coating the paste formulation on atleast one side of the single layer substrate.
 2. The method of claim 1,wherein the paste formulation further comprises a pore former, carbon,graphite or Ni powder.
 3. The method of claim 1, wherein the singlelayer substrate comprises a thin conductive material.
 4. The method ofclaim 3, wherein the thin conductive material comprises a perforatedmetal foil or sheet, metal mesh or screen, woven metal, or expandedmetal.
 5. The method of claim 4, wherein the thin conductive materialcomprises a nickel plated perforated foil.
 6. The method of claim 1,wherein the single layer substrate comprises a three dimensionalmaterial.
 7. The method of claim 6, wherein the three dimensionalmaterial comprises a metal foam or metal felt.
 8. The method of claim 1,wherein the sulfur is present in the paste formulation in the amount offrom 0.25 to 1.5% by weight.
 9. The method of claim 1, wherein the pasteformulation is coated on both sides of the substrate.
 10. The method ofclaim 1, wherein the sulfur comprises elemental sulfur.
 11. The methodof claim 1, wherein the active iron material comprises iron metal, aniron oxide material, or a mixture thereof.
 12. The method of claim 11,wherein the iron oxide material comprises Fe₃O₄.
 13. The method of claim1, wherein the polyvinyl alcohol binder comprises polyvinyl alcohol thatis hydrolyzed between 98.5 and 100%.
 14. The method of claim 1, whereinthe polyvinyl alcohol binder comprises polyvinyl alcohol that ishydrolyzed between 99 and 100%.
 15. The method of claim 1, wherein thepolyvinyl alcohol binder comprises from 2.5 to 4 wt % of the ironelectrode.